1. Field of the Invention
The present invention relates to multi-window wavelength division multiplexers (MWDMs) and filters and, in particular, to MWDMs with uniform spectral response within passbands using unbalanced Mach-Zehnder interferometers and Fabry-Perot filters.
2. Discussion of the Related Art
With existing fiber optic networks, there is often the need to increase information transmission capacity. However, both physical and economic constraints can limit the feasibility of increasing transmission capacity. For example, installing additional fiber optic cable to support additional signal channels can be cost prohibitive, and electronic system components may impose physical limitations on the speed of information that can be transmitted. The use of wavelength division multiplexers (WDMs) provides a simple and economical way to increase the transmission capacity of fiber optic communication systems by allowing multiple wavelengths to be transmitted and received over a single optical fiber through signal wavelength multiplexing and demultiplexing. In addition, WDMs can be used in fiber optic communication systems for other purposes, such as dispersion compensation and noise reduction.
WDMs can be manufactured using, for example, biconical tapered fusion (BTF) technology. Typically, two optical fibers are fused together along an interior portion to form a fused-fiber coupler, so that light of two wavelengths (e.g., 1310 nm and 1550 nm) entering the input ports of the first and second fibers, respectively, are multiplexed onto a single fiber. The coupling ratios for the two channels (the signals at 1310 nm and 1550 nm) exhibit complementary sinusoidal behavior for amplitude as a function of frequency within the passband of the WDM, with each channel having one or more peaks (or windows) within the passband. Information carried by the two signals along the single fiber is then demultiplexed at the WDM outputs.
Multi-window WDMs (MWDMs) have two or more peaks of amplitude as a function of frequency (or operational windows) for each channel within a passband. MWDMs can also be made using BTF technology by twisting two optical fibers together, fusing the center portion together, and pulling the fibers until a desired multi-window transmission spectrum appears at a monitored fiber output port. MWDMs can also be made using unbalanced Mach-Zehnder interferometers (MZIs), as disclosed in commonly-owned U.S. patent application Ser. No. 09/034,895, entitled "Fused-Fiber Multi-Window Wavelength Division Multiplexer Using Unbalanced Mach-Zehnder Interferometer", filed Mar. 3, 1998, which is incorporated by reference in its entirety.
FIG. 1 shows a fused-fiber MWDM 10 formed from an unbalanced MZI, which uses identical first fused-fiber coupler 11 and second fused-fiber coupler 12, coupled together by connecting fibers 13 and 14 having different optical path lengths. Fused-fiber couplers 11 and 12 can be formed by heating and axially stretching two optic fibers to form a fused coupling region. Broadband light at two wavelengths, entering coupler 11 or 12 at input ports 15-1 and 15-2 or 16-1 or 16-2, respectively, couple onto and travel along the fused coupling region. The light then decouples and exits coupler 11 or 12 at output ports 17-1 and 17-2 or 18-1 and 18-2, respectively. Couplers 11 and 12 are typically 3-dB couplers, so that power entering an input port (e.g., 15-1) is equally divided between two output ports (e.g., 17-1 and 17-2). The different optical path lengths of connecting fibers 13 and 14 result in the two optical signals arriving at the next coupler stage at different times, so that optical signals propagating through the connecting fibers are phase-shifted.
FIG. 2 shows the transmission spectrum from output ports 18-1 and 18-2, which consists of alternating peaks and nulls. Due to the optical path length difference between the two connecting fibers 13 and 14, different constructive and destructive interference occurs at different wavelengths, resulting in the spectrum of FIG. 2. Solid line 21 represents the amplitude as a function of frequency of the output signal from one transmission channel (e.g., output port 18-1), while dashed line 22 represents the amplitude as a function of frequency for the simultaneous output signal from the other transmission channel (e.g., output port 18-2).
The channel spacing .DELTA..lambda. of an MWDM, defined as the wavelength separation between the transmission peak wavelengths of two adjacent channels, as shown, for example, by the separation of adjacent peaks 23 and 24, can be expressed by equation (1) below: ##EQU1##
where .lambda. is the central wavelength, and .DELTA.L is the optical path length difference between connecting fibers 13 and 14, .DELTA.L being equal to n.sub.1 l.sub.1 -n.sub.2 l.sub.2, where n.sub.1 and n.sub.2 are the respective refractive indexes and l.sub.1 and l.sub.2 are the respective lengths of the two connecting fibers 13 and 14. The window spacing of the MWDM, which is normally twice the channel spacing, is defined by the wavelength separation between two adjacent transmission peak wavelengths from a channel, as shown, for example, by the separation of peaks 21 and 23. By increasing the optical path length difference AL, the channel separation is decreased so that more wavelengths can be transmitted on a single fiber, thereby forming devices known as dense WDMs.
The spectrum shown in FIG. 2 can be approximated mathematically according to equation (2) below: ##EQU2##
where .lambda. is the central wavelength, and .DELTA..lambda. is the channel spacing given by equation (1) above. As seen from equation (2) and FIG. 2, the spectral response within a passband of MWDM 10 is curved, i.e., it drops off sinusoidally from both sides of the central wavelength. This results in signals within the passband subject to non-uniform attenuation or gain. Ideally, in an optical communication system, a flat or uniform spectral response within the passband is preferred because the modulated optical signal can maintain a better waveform in a high data rate system. In addition, the light signal can remain at approximately the same power level if the signal wavelength varies within the passband. A flat spectral response is especially critical with dense WDM (DWDM) systems, in which groups of as much as 64 wavelengths are simultaneously transmitted in a fiber.
Accordingly, a structure and method are desired which achieves a flat spectral response within passbands of MWDMs.